For patients diagnosed with disc degeneration, surgical operations are performed more and more often. The most common operation for these patients today is fusion, where an ossified connection of vertebrae is obtained. Also, metallic connection devices can be used. Movability then ceases between the vertebrae in question but the patient will become free from pain. As the patient becomes more active and movable, the segments above and below the fused region will, however, be subjected to greater strains. The risk of new symptoms from surrounding segments thereby increases.
As an alternative to fusion, disc implants have been presented. A known disc prosthesis generally consists of two mutually articulated plates that are positioned between two vertebrae instead of the disc. The positioning of a disc implant results in eliminating the disc that causes pain, reinstating the distance between the vertebrae and reinstating movability between them.
In order to obtain sufficient certainty against a disc implant moving, over time, in an undesired manner from the intended position between two vertebrae, two mutually articulated plates of a previously known prosthesis are provided with different kinds of projecting engagement means, such as fin-shaped elements, pointed elements, pins and like plural projections for the engagement with the meeting surfaces of the vertebrae.
An operative method that is used for inserting a disc implant requires positioning of the prosthesis from the abdomen side in order to allow access to the vertebral column from the front. The disc to be replaced is cleared out, whereby the vertebrae are drawn apart with the aid of tension pliers.
After using instruments for shaping grooves in the surfaces of vertebrae for the cooperation with possible projecting protrusions for the purpose of achieving a correct position for the disc prosthesis, the latter is now to be positioned.
According to today's methods, this is achieved by placing the prosthesis on a holder and hammering it in with great force between the vertebrae, guided by the prepared grooves. This step in the operation is very problematic, since the vertebrae tend to be drawn against each other and then it is often difficult to implant the prosthesis between the vertebrae. For this reason, implantation by hammering is risky.
When the prosthesis is finally positioned, which is verified with X-ray radioscopy, tension pliers are used in order to again span apart the vertebrae, and thereby the disc plates of the prosthesis, in order to be able to position a joint detail between these plates. Also this step is troublesome and sometimes laborious.
When using prostheses that are completely assembled which include the joint detail, this second step is not necessary. Such prostheses are, however, thicker and thus, more troublesome to position between the vertebrae. Sometimes, unfortunately, damage to the vertebrae can occur during the positioning of the prosthesis. Such damage can be serious and have serious consequences. For that reason, the surgical operation puts great demands on the skill and experience of the surgeon.
When, finally, everything is place, the operation is terminated and a final X-ray is made. Sometimes, it is discovered that the disc prosthesis is positioned in such a way that it is not placed exactly on the middle line or is not in a proper position. The possibilities of adjusting the position are at this stage almost none.
Since the disc prosthesis rests on the brittle covering plate of the vertebra, the prosthesis must have maximal size in order to support on a relatively strong peripheral rim of the vertebra. Exact positioning is therefore very essential. Patient having osteoporosis are therefore often disqualified for this type of surgical operation depending on lack of congruence between the parts of the disc prosthesis and the vertebra.
Incorrect positioning results in risk of uncontrolled separation and repositioning of the vertebrae.